Metal Oxide Varistor Thermal Cut-Off Fuse Silver Electrode

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Solution Overview

Problem

Conventional metal oxide varistors (MOVs) face issues with thermal runaway and combustion due to limited thermal conductivity, as existing thermal cut-off fuses often fail to disconnect power quickly enough to prevent overheating.

Innovation Solution

Incorporating a silver electrode area with high thermal conductivity to facilitate rapid heat transfer to a thermal cut-off fuse, which interrupts current flow when overheated, thereby preventing thermal runaway.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a thermal cut-off fuse is wired with the MOV and positioned adjacent one face of the MOV, then the thermal cut-off fuse can melt and open the lead to interrupt current flow, but the thermal conductivity of epoxy and air is limited and surrounding air can cool down the heat, causing the thermal fuse to cut off power just after the MOV becomes failed and burns

Engineering Contradiction:

Improvethermal cut-off functionVSAvoidpower cut-off speed

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

A silver electrode area is introduced as an intermediary thermal conduction path between the MOV body and the thermal cut-off fuse. The silver electrode area has high thermal conductivity, serving as an efficient heat transfer medium that rapidly conducts heat from the MOV body to the thermal cut-off fuse, enabling the fuse to melt and interrupt current flow before the MOV overheats and burns.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The thermal conductivity parameter of the heat transmission path is significantly improved by replacing the conventional epoxy and air medium with a silver electrode area. The silver electrode area provides a high thermal conductivity path, fundamentally changing the heat transmission efficiency and enabling rapid thermal response of the cut-off fuse.

Inventive Principle:

Principle #35Parameter changes

2Duration of action of moving object

If the MOV is used for a long time, then the original high impedance of the resistance becomes low and leakage current occurs, but the leakage state deteriorates gradually and the leak current forms a leak point with material melted and producing a short circuit point, leading to overheating and combustion

Engineering Contradiction:

Improveservice lifeVSAvoidthermal runaway and combustion

Core Design Contradiction:

Duration of action of moving objectVSObject-affected harmful factors

Solution Approach 1:

The thermal cut-off fuse is pre-positioned adjacent to the MOV body with a silver electrode area providing a dedicated thermal conduction path. This preliminary arrangement ensures that when the MOV degrades over time and generates excessive heat from leakage current, the heat is rapidly conducted to the fuse, which melts and interrupts current flow before thermal runaway and combustion can occur.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The thermal cut-off fuse acts as a protective cushioning mechanism against thermal runaway. By positioning the fuse with direct thermal contact through the silver electrode area, the system is pre-prepared to absorb and respond to thermal stress, preventing the harmful progression from leakage current to combustion even after long-term usage.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution enables quick and effective power disconnection, reducing the risk of MOVs overheating and combustion by leveraging the silver electrode's low inductance and high thermal conductivity to rapidly transfer heat to the thermal cut-off fuse.

Implementation Method 1

the MOV has a lower inductance, and accordingly, enables the MOV to have a high and sound thermal conductivity. Hence, the MOV can fleetly and perfectly transfer heat to the thermal cut-off fuse

Methodology Applied

Scientific EffectThermal conduction: Conduction (thermal)

Implementation Method 2

The thermal cut-off fuse has a predetermined melting point at which the thermal cut-off fuse melts and interrupts current flow therethrough

Methodology Applied

Scientific EffectMelting: Melting

Implementation Method 3

The MOV body heats up when exposed to voltage strikes

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Data Source

PatentUS7598840B2Metal oxide varistor having thermal cut-off function

Publication Date: 2009.10.06 CENTRA SCI HLDG

AI summary

A MOV includes a MOV body, a first lead, a thermal cut-off fuse, a second lead and a silver electrode area. The MOV of the present invention employs the silver electrode area formed on and electrically coupled to the MOV body, thus the MOV has a lower inductance, which accordingly, enables the MOV to have a high and sound thermal conductivity. Hence, the MOV can fleetly and perfectly transfer heat to the thermal cut-off fuse in case of over-voltages, thus enabling the thermal cut-off fuse to cut off the power more quickly.